As the demand for computer-based voice, video and data services increases, so does the demand for higher bandwidth. Networked technologies such as Integrated Services Digital Networks (ISDN), frame relay, Switched Multimegabit Digital Service (SMDS), Asynchronous Transfer Mode (ATM), satellite data communications systems, wireless communications systems and others have all been developed to meet this demand for bandwidth. To make these services universally available requires either a new communications network infrastructure, or significant modifications to the existing one. None of these systems offers users the ability to transmit and receive data at high speeds and low cost by using regular analog channel connections to the public switched telephone network.
A common low cost alternative for transmitting data is the conventional modem, which is designed to modulate and demodulate signals between a user's digital devices, and analog channels connected to the public switched telephone network. Conventional modems offer users the ability to send and receive data at speeds of up to 33.6 kbit/s, while a newer generation of modems offer users the ability to receive data at speeds of up to 56 kbit/s. Due to limitations inherent in ordinary analog channels, this is the maximum achievable data rate for a conventional analog modem.
An option that is open to a user who desires higher data rates includes making use of switched digital services offered by interexchange carriers and local exchange carriers. These services make it possible for a user to dial up point-to-point digital connections whose bandwidth ranges from 56 kbit/s to 3 Mbit/s and beyond. Examples of these switched digital services include Switched 56, Switched 64, Switched 384, Switched 1536, and ISDN Multirate. However, the use of these switched digital services requires specialized digital access lines to the user's premises. The use of these digital access lines is commonly sold at a premium over the cost of a regular analog telephone line connected to a user's premises.
Another option open to a user who desires higher data rates is to make use of inverse multiplexing over digital data channels. Inverse multiplexing is a mechanism for aggregating multiple independent data channels across a network to create a single higher rate data channel. The equipment that performs inverse multiplexing is called an inverse multiplexer. A user desiring a higher data rate would use an inverse multiplexer to aggregate multiple calls that have been set up using the switched digital services described above. For example, if six different independent 56 kbit/s data channels are established between points A and B in a network, an inverse multiplexer can be used to combine these channels to create a single 336 kbit/s (ie. 6.times.56=336) data stream. Likewise, 64, 384 and 1536 kbit/s channels can be inverse multiplexed together. Inverse multiplexers are sold by equipment vendors such as Ascend Communications, Inc, Nortel.TM., and U.S. Robotics.TM..
The most common example of inverse multiplexing that is made available to users is the ISDN Basic Rate Interface (BRI). An ISDN BRI specifies an ISDN line that has two 64 kbit/s data and voice (B) channels, and one 16 kbit/s signalling (D) channel. The two B channels are inverse multiplexed together to provide the user with a single high-speed 128 kbit/s channel. However, as was the case with switched digital services, specialized digital access lines to the user's premises are required to take advantage of the capabilities of inverse multiplexing. In addition to the extra costs levied by telephone companies to use digital access lines, a further disadvantage is that these digital access lines are not available in every area where local phone service is provided.